1. Field of the Invention
The present invention relates generally to an improved ball nut for use in combination with ball screws to give a positioning assembly. More particularly, it relates to such a ball nut which incorporates flexibility while being suitable for use with relatively large diameter ball screws and with relatively heavy loads. Most especially, it relates to such a ball nut and positioning assembly that provides a high repeatability in positioning.
2. Description of the Prior Art
There is a growing market and ever increasing demand for ball screw assemblies that have high repeatability. Repeatability refers to the ability to position along the axis of the ball screw, make a move and then return to the previous position very accurately. Backlash in the ball screw assembly reduces the accuracy of the system's repeatability. Backlash is the `play` or `lost motion` in the ball screw assembly. Preloading ball nuts onto ball screws is a common practice in the ball screw industry.
Conventional ball screw assemblies use a ball screw with a helical groove along its length, and a ball nut with a mating groove that interacts with the lead screw groove to form a race for ball bearings positioned between the grooves of the lead screw and the ball nut. The ball bearings move in rolling contact in the grooves as the lead screw and the ball nut are rotated with respect to one another. A return path is provided as part of the ball nut so that the ball bearings are constantly recirculated in the race as the lead screw and ball nut continue to rotate with respect to each other.
There are three basic methods for removing backlash from the ball screw assembly currently in use. A technique known as the `solid preload` achieves its goal by creating an interference fit between the ball nut and the ball screw by filling the ball race with ball bearings that are slightly larger than the actual space in the ball race. Another solid preloading technique is to force two separate ball nuts either apart or together with a solid member. Solid preloading is almost exclusively limited to use with high precision ground ball screw assemblies.
The second method is applied to both high precision grade assemblies and the commercial grade called `rolled thread`. This method involves two separate ball nuts assembled to the ball screw with a preloading device. Most commonly, this device is a set of collars and springs that bias the separate nuts apart and, at the same time, holds them in relationship to each other. These designs create a somewhat flexible assembly and are usually used with rolled thread ball screw assemblies.
The third method employs the spring nature of the hardened ball nut material. A helical flexure is created in the body of the ball nut within its ball circuit. This flexure results in the ball circuit actually becoming a dynamic preloading coil spring. This spring design allows the ball nut to run along the lead screw with the coil spring constantly adjusting to oscillating motion of the lead screw caused by irregularities in the lead screw as translated through the ball bearings while continually applying a preload to the ball screw assembly. This design is presently applied exclusively to the rolled grade ball screw assemblies. This design is the subject of my prior U.S. Pat. No. 4,896,552, issued Jan. 30, 1990.
The above three preloading methods all have one of two drawbacks that limit their effectiveness. The solid preload lacks any flexibility. It cannot self adjust to variations in the ball screw thread and can therefore only be used with very expensive, high precision ball screws with any reliability. Variations in the ball screw thread are found in all grades, but are greater in the commercial rolled grade than in the precision grade. The primary advantage of the solid preload is in its stiffness, which makes it suited for applications producing a high load on the ball screw.
The preloading collar double nut design offers some flexibility through the spring(s) in the collar arrangement. However, the small gains in flexibility are offset by the loss of stiffness in the ball nut assembly. Because the spring in this unit can supply only a marginal amount of flex, these assemblies tend to run roughly, and not well at all when applied to small diameter lead ball screw assemblies.
The third method of turning the ball circuit into a preloading coil spring offers superior flexibility over both of the above designs. This design works very well where others do not. However, this design lacks stiffness. Such a flexible coil spring cannot offer high stiffness. This design is therefore limited to use with low to moderate loads, where stiffness is less necessary.